You'll see both 'wreaked havoc' and 'wrought havoc' in many newspaper articles today, discussing the unrest in some cities. Which is correct? Strictly speaking, both are. 'Wrought' is often erroneously thought to be the past tense of 'wreak', but it isn't; it is an archaic past tense form of the verb 'to work' (as in 'wrought iron', for instance). The reason why both 'wreaked havoc' and 'wrought havoc' are ok is because the expression 'to work havoc' used to be common, so, of course, its past tense was 'wrought havoc'. Now, however, those newspapers and websites which use 'wrought havoc' (such as this BBC page) never use 'work havoc' in the present tense.